Approximately like this:
curdata = get(handles.uitable, 'data');
for K = 1 : size(curdata, 1)
curstring = curdata{K, 3); %for column 3
curstring{end+1} = 'yellow'; %adding yellow as a choice
curdata{K,3} = curstring;
end
set(handles.uitable, 'data', curdata)
